U.S. Pat. No. 5,233,911 discloses a typical tandem brake booster through which a force is developed to effect a brake application. In such a brake booster, the interior is divided into first and second front chambers and first and second rear chambers by corresponding first and second walls. The first front chamber is externally connected to a first source of fluid pressure by a conduit and internally directly connected to the second front chamber and to the first and second rear chambers by various passages in a hub associated with the first and second walls. A control valve located in the hub is positioned such that the first fluid pressure is communicated in the first and second rear chambers to define a rest or ready position. In responsive to an input force, the control valve is positioned such that a second fluid pressure is presented to the first and second rear chambers to create a pressure differential across the first and second walls. This pressure differential acts on the first and second walls to develop an output force which is provided to a master cylinder to assist in effecting a brake application. On termination of the input force, a spring returns the control valve to a rest or ready position of the first mode of operation as the second fluid is evacuated from the first and second rear chambers to allow the first fluid pressure to again be present in the first and second rear chambers. This type vacuum brake booster functions in an adequate manner to provide an assist in effecting a brake application.
U.S. Pat. Nos. 4,409,885, 4,942,738 and 5,329,769 disclose brake boosters and master cylinder arrangements wherein at least a portion of the master cylinder housing is positioned within the interior of the booster housing to define an integrated brake assembly. Such brake boosters operate in a satisfactory manner but because of their overall size they still occupy considerable under the hood space when installation in a vehicle.
In a typical brake system, fluid pressure is supplied to separate brake circuits after passing through a proportioning valve to assure that the fluid pressure supplied to corresponding brakes i.e. right and left front brakes and right and left rear brakes, is identical such as disclosed in U.S. Pat. Nos. 4,753,487 and 4,908,522. In such brake systems if a brake circuit should become inoperative or develop a leak, it is possible that different fluid pressures will actually be presented to the individual brakes during a brake application. This situation is undesirable since it is possible to create a skid condition under certain conditions.